REPORT METEOROLOGY AND CLIMATOLOGY No. 117, 2015 The Meteorological Auto Code (MAC) and Numerical Weather Prediction (NWP) at SMHI Lennart Bengtsson, Nils Gustafsson1, Bo Döös2, Daniel Söderman, Lars Moen3, Thomas Thompson4, Paul Jakobsson, Gunnar Bleckert, Ann-Beate Henriksson, Bo Lindgren5 and Per Kållberg Front: Model equations and time-stepping scheme of the first SMHI NWP model on BESK and initially on Datasaab D21 ISSN: 0347-2116 © SMHI REPORT METEOROLOGY AND CLIMATOLOGY No. 117, 2015 The Meteorological Auto Code (MAC) and Numerical Weather Prediction (NWP) at SMHI Lennart Bengtsson, Nils Gustafsson1, Bo Döös2, Daniel Söderman, Lars Moen3, Thomas Thompson4, Paul Jakobsson, Gunnar Bleckert, Ann-Beate Henriksson, Bo Lindgren5 and Per Kållberg 1Correspondig author, e-mail: [email protected] 2Deceased 2010 3Deceased 2006 4Deceased 2015 5Deceased 2005 Abstract Sweden was a pioneering country in the development of Numerical Weather Prediction (NWP). The worlds first operational numerical forecast was produced already in 1954 by the Interna- tional Meteorological Institute in Stockholm. SMHI started a bit later, but in 1961 a long term program for development of NWP was initiated. The activities grew gradually during the 1960’s and resulted in a core component for the SMHI forecast services. An early challenge was to overcome the limited computational resources with slow computational speed, small memory size and primitive software support. It was necessary to compensate for these limitations with dedicated work and creativity. A core component in this work was the software system MAC (Meteorological Auto Code) that was developed by the NWP group at SMHI. The MAC system is described in detail in this report and it included all computational software needed for the weather service, for example numerical models, objective analysis techniques, automatic data extraction, quality control of observations as well as forecast products in graphical or digital form. We hope that this report will provide the younger generation with some insight into the con- ditions for development of NWP during the 1960’s. Sammanfattning Sverige var ett foreg¨ angsland˚ inom numeriska vaderprognoser¨ och den allra forsta¨ operativa vader-prognosen¨ gjordes redan 1954 pa˚ det Internationella Meteorologiska Institutet i Stock- holm. SMHI kom igang˚ senare, men 1961 startade man ett langsiktigt˚ program for¨ NWP (nu- merical weather prediction). Projektet vaxte¨ gradvis under 1960-talet och blev sa˚ smaningom˚ en central komponent i SMHIs prognostjanst.¨ En utmaning under de tidiga aren˚ var de begransade¨ dataresurserna med primitiv programvara, och med dagens matt˚ begransat¨ minnesutrymme och lag˚ berakningshastighet.¨ For¨ att kompensera dessa brister kravdes¨ bade˚ beslutsamhet och ett stort matt˚ av kreativitet. Som en central komponent i arbetet utvecklade NWP-gruppen da- torsystemet MAC (Meteorological Auto Code) som har¨ beskrivs i detalj samt ocksa˚ alla de berakningsprogram¨ som kravdes¨ for¨ prognostjansten.¨ Detta inkluderade olika prognosmodeller, analys samt program for¨ databehandling och observationskontroll samt produktion av prognos- resultaten i grafisk eller digital form. Det ar¨ var˚ forhoppning¨ att foreliggande¨ artikel skall ge den yngre generationen en inblick i hur det var att syssla med NWP under 1960-talet. Contents 1 Introduction 1 2 A brief history of early Swedish computers 2 3 The challenge of numerical weather prediction in the early 1960’s 4 4 Modelling and data assimilation 4 4.1 Introduction . 4 4.2 The model development . 5 4.3 Objective analysis of meteorological fields . 6 5 Building a computer based forecasting system 7 5.1 Decoding and quality control of observational data . 7 5.2 Post-processing of data . 10 6 Technical structure of the forecasting system 11 6.1 Constraints due to the hardware . 11 6.2 Programming and structuring of a complex software system . 11 6.3 Handling of grid point fields in model and analysis calculations . 12 6.4 Disposition of the small memory in model calculations, bit packing . 12 6.5 Connecting grid point space and observation space in the analysis - the need for efficient input/output . 13 6.6 Use of the D21 loud-speaker for debugging . 13 7 Back to the future 14 8 Acknowledgement 14 9 References 15 Appendix A: Master subroutine for the barotropic vorticity advection model on D21 16 Appendix B: People and social life from the perspective of a newcomer 20 1 1 Introduction Numerical weather prediction started very early in Sweden under the auspices of the Interna- tional Meteorological Institute (IMI) supported by the Swedish Military Weather Service. IMI was established in 1951 under the leadership of Carl-Gustav Rossby and reported directly to the Ministry of Education but from 1955 to Stockholm University. The worlds first operational numerical forecast ever was carried out in 1954 using the Swedish-built computer BESK. SMHI started later but in 1961 a small research and development group for NWP was set up. This was initiated and initially led by Bo Do¨os.¨ Do¨os¨ had considerable experience as he had been one of the leading scientists at IMI that developed a comprehensive NWP-system used for a real-time prediction in 1955 by the Swedish Air Force. SAAB (Svenska Aeroplan AB) was during the period in the process to develop a more advanced computer, D21, than the computer used by the Rossby group. It was faster than BESK and had a larger memory. It was also technically more advanced and used more modern hardware components than BESK, but it still used the same primitive systems for input and output. Compared to the computer systems of today, more than half a century later, the D21 was also utterly primitive, in particular with regard to software. There was no operating system and no compilers available when SMHI started to use D21. There was no hardware available for floating point calculations, hence all programming had to be done with fixed point numbers and in machine language. Special users oriented compilers and operating systems had to be developed by the group at SMHI. The D21 at SMHI initially had a memory of only 12288 24 bit words or 36 kilobytes. The computer had some 40 different operations. There were special codes for indirect addressing and for double word length operations. The time for a typical operation like addition was 9.6 microseconds (µs), some 3-4 orders of magnitude slower than today’s portable computers and mobile telephones. The requirements on the programmers were substantial and a major objective was to write efficient codes that used a minimum of memory space. The main objective of this paper is to explain the structure of the code and the system that was developed from scratch. We will refer to this system as the Meteorological Auto Code (MAC) system. The MAC system was used at SMHI until the late 1970’s when it was replaced with a modern Fortran based system. It was also implemented in a few other countries such as Finland and Czechoslovakia. The structure of the code was based on an octal number system using the numbers 0-7 for instructions, data and meteorological parameters but including 8 to represent relative addressing and 9 to indicate individual programs. As will be shown in this article this was found to be very useful. The individual programs were arranged in four separate groups; data handling including decoding and error control of individual observations, programs for analysing the data into grid point fields, numerical modelling codes and programs for presenting the result in different ways. In the following sections these systems will be described and examples will be provided. A short history will also be given on the state of computing in Sweden in the 1950’s. Appendix A includes a MAC subroutine for the barotropic model and Appendix B provides some remarks on people and social life. 1 2 A brief history of early Swedish computers The urgent need for computers (’matematikmaskiner’) for Swedish research and for the Swedish defense, was recognized early in the 1940’s. The Swedish Board for Computing Machinery (Matematikmaskinsnamnden,¨ MMN) was established through a decision by the Government on the 26th of November 1948. Included among the members of the MMN were representatives from universities, academia and the Swedish defence. MMN was given the task to follow the international development in computing and to investigate possibilities to provide computers for the Swedish needs. Initially, the possibilities for importing computers from the U.S. were explored. Several experts were sent to study ongoing computer projects at U.S. universities and several commercial providers of computers in the U.S. were contacted. From the contacts with U.S. Governmental authorities it was realized, however, that the chances of getting a license for export of computers to Sweden were very small. The logical and brave recommendation by MMN in January 1949 was to initiate construction of computers in Sweden. This recommen- dation was very important for Swedish activities in computing science, computer industry and numerical weather prediction for decades to come! The MMN initiated a strategy for the construction of Swedish computers along two different time scales, one fast line of development (months) based on standard telephone relays and a more long term (2-3 years) project for development of electronic computers based on vacuum tubes. One early computer BARK (Binar¨ Aritmetisk Rela¨ Kalkulator) was based on standard tele- phone relays and it was completed already in January 1950. It had a memory with 100 change- able registers and 200 fixed registers, a computing accuracy corresponding to 7 decimal digits (a 24 bits mantissa and a 6 bits exponent) and the computing speed was approximately 100 ms for addition and 250 ms for multiplication. BARK was utilized by various users on a commercial basis from June 1950. Among the application areas were development of numerical techniques, aerodynamics, ballistic calculations for the defense and sediment age estimations.